Cement compositions give concrete and other hardening products excellent in strength and durability and, therefore, are used widely in such fields of application as outer walls of buildings and architectural structures. Such cement compositions generally contains an admixture for a cementitious composition so that the air entraining properties and fluidity thereof may be enhanced. In recent years, the importance of admixtures for a cementitious composition has been recognized and a number of technological innovations have been made. The admixture for a cementitious composition plays a role in securing the fluidity and workability of a cement composition by exhibiting its dispersing ability even when the water content in the cement composition is reduced, in realizing improvements in durability and strength owing to water reduction and in providing a good cement composition with retained dispersibility not changing with time.
In the recent concrete industry, it is strongly demanded that concrete structures have improved durability and strength. Accordingly, reduction of the unit water content in cement compositions is now an important subject. In particular, a large number of polycarboxylic acid cement dispersants have been investigated since higher water reducing ratios can be attained with them as compared with the conventional cement dispersants of naphthalene and the like. The use of such a polycarboxylic acid-based cement dispersant can lead to increase in air entraining properties and in bubble size. Thus, for improving the freeze-thaw resistance by means of entrainment of minute bubbles of good quality, it is used in combination with an air-entraining (AE) agent.
However, it is a drawback with the combined use of a polycarboxylic acid cement dispersant and an AE agent that the amount of air entrained progressively increases as concrete kneading is continued and this makes it difficult to attain the desired concrete strength. The conventional antifoaming agents are indeed effective in temporarily reducing the amount of air entrained but, in this case, too, continued kneading allows the amount of air entrained to increase progressively. In view of the on-site workability, it is difficult to add an antifoaming agent just prior to placing concrete and, therefore, it is required that an antifoaming agent be added in the production of concrete. In the United States, in particular, the addition of an AE agent is essential as a measure to cope with the problems arising from freezing and thawing and, furthermore, the increase in the amount of air entrained is a serious problem since kneading is continued for a long period of time during transportation.